1. Field of the Invention
The present invention relates to an image-taking apparatus, and to a camera and a camera system incorporating it. More specifically, the present invention relates to an image-taking apparatus provided with a taking lens system that offers a wide angle of view and simultaneously offers high definition in a central region of a telephoto image, and to a camera and a camera system incorporating such an image-taking apparatus.
2. Description of the Prior Art
Common image-taking apparatuses adopt taking lens systems that have satisfactorily corrected distortion so that there is only a small variation in image magnification between in a central and a peripheral portion of the light-receiving surface (i.e., the image-sensing surface) of an image sensor. Apart from these, there also exist taking lens systems that are designed to produce large negative distortion. Such taking lens systems, of which an example is a fish-eye lens having a total angle of view as wide as about 180°, are used to obtain super-wide-angle images. With these, by intentionally producing negative distortion, a peripheral portion of an image is greatly compressed relative to a central portion thereof.
Non-patent references 1 and 2 cited below propose wide-angle lenses for use in an eye of a robot (hereinafter, such a lens will be referred to as a “fovea lens”) which have characteristics close to those of the latter of the two types of taking lens systems described above. All such fovea lenses are designed to mimic human vision. In the human eye, there is a region called the “fovea centralis,” inside which optic cells (nerve cells sensitive to light) crowd more densely than outside. Although the eyesight is poor outside the fovea centralis, the movement of the eyeball permits the region of interest to be always observed with the fovea centralis. This is a highly efficient structure from the viewpoint of minimizing the amount of information handled. A fovea lens, which mimics this structure of the fovea centralis, has high definition only in a limited central region so as to reduce the total amount of input image information and thereby enhance the image processing speed. Another big feature of a fovea lens is that it has a far higher image magnification in a central portion of the image-sensing surface than a fish-eye lens or the like. As a result, a fovea lens, despite being a single-focal-length lens having no optical zooming capability, makes it possible to simultaneously obtain a wide-angle image as a whole and a telephoto image in a central portion of that image.
Non-patent reference 1
Y. Kuniyoshi, N. Kita, K. Sugimoto, S. Nakamura, T. Suehiro: “A Foveated Wide Angle Lens for Active Vision,” Proc. IEEE Int. Conf. Robotics and Automation, pp. 2982-2988, 1995 (searched on line on Mar. 26, 2003 on the Internet, URL: http://www.is.aist.go.jp/acac/publications/ICRA95.ps).
Non-patent reference 2
Y. Suematu, H. Yamada, T. Ueda: “A Wide Angle Vision Sensor with Fovea—Design of Distortion Lens and the Simulated Images—,” IEEE International Industrial Electronics Conference, USA, pp. 1770-1773 (1993).
Attempting to obtain an image without distortion even in a peripheral portion thereof with a wide-angel lens having a total angle of view over 60° results, due to an increased number of lens elements and other factors, in complicating the construction of the optical system, leading to higher costs. Attempting to record a super-wide-angle image with uniformly high definition results in handling a huge amount of information, necessitating an expensive high-resolution image sensor. Such an increase in the amount of information to be handled is undesirable from the viewpoint of quick transfer of data when image information is exchanged in cellular phones, personal information assistants, and the like, which are becoming increasingly popular these days,
In super-wide-angle lenses (or fish-eye lenses) that are designed to intentionally produce large negative distortion, one method to obtain an enlarged image in a central portion of the overall image is with optical zooming, whereby part of the optical system is moved, and another is with electronic zooming, whereby image data is interpolated through image processing. The former method complicates the construction of both the optical system and the drive mechanism, leading to higher costs, and the latter method degrades image quality.
On the other hand, conventional fovea lenses are composed of 7 to 11 lens elements, and thus have a large-scale optical system. Moreover, all those lens elements are glass lens elements, resulting in high costs. Moreover, they suffer from large residual aberrations and thus unsatisfactory imaging characteristics. For these reasons, conventional fovea lenses are unsuitable for achieving high image quality, compactness, and low costs. Moreover, they all have an f-number as low as f4, and it is difficult to give them large apertures.